This invention is directed to a seal for use with rotating members such as shafts, and more particularly to a labyrinth seal having radially disposed sealing elements. The invention is also directed to a method for installing the seal in equipment, and particularly to a method for installing the seal as a replacement for an axial labyrinth sealing arrangement originally present in equipment such as a steam turbine.
FIGS. 1A and 1B illustrates portions of a conventional high pressure turbine 10. Turbine 10 includes a rotor 12 having end portions which are configured as shaft ends 14 and 16. Rotor 12 also has an axial bore, as illustrated, to remove impurities which collect along the axis during forging of the rotor and to permit inspection for imperfections. Shaft ends 14 and 16 are rotatably supported by bearings 18. Turbine 10 also includes a housing, known as a "cylinder" in the turbine art due to its generally cylindrical configuration. Reference number 20 identifies the cylinder of turbine 10. Cylinder 20 includes an upper shell 20A and a lower shell 20B, which are connected by bolts (not illustrated).
During operation, superheated steam from a boiler (not illustrated) or steam generator (not illustrated) is conveyed via control valves (not illustrated) to inlet pipes 22, which empty into cavity 24 within cylinder 20. A channel 26 conveys the superheated steam to a cavity 28. Superheated steam escapes from cavity 28 through apertures in an annular nozzle plate 30 which is affixed to cylinder 20. A stator wheel 32 having fan-type blades is also affixed to cylinder 20. Rotor 12 has annular grooves (blade roots 34) with a latching configuration to retain a pair of rotor wheels 36, one on either side of fixed stator wheel 32. Rotor wheels 36 have fan-type blades which are oriented in a direction opposite from the orientation of the blades of stator wheel 32. Nozzle plate 30 and the stator and rotor wheels 32 and 34 are known collectively as a control stage, identified in FIG. 1 by reference number 38. Superheated steam escaping outward from cavity 28 through nozzle plate 30 first encounters the inner rotor wheel 36 and thereby imparts an impulse to rotor 12. After the direction of the steam is corrected by fixed stator wheel 32, the steam encounters the second rotor wheel 36 and imparts an additional impulse to rotor 12. The steam entering expansion chamber 40, while still superheated, nevertheless has a pressure and temperature significantly less than the pressure and temperature of the steam in cavity 28. Control stage 38 extracts mechanical energy from the steam.
A blade ring 42 is affixed to cylinder 20 and supports a plurality of stator wheels 44, which are interleaved with rotor wheels 46 affixed to rotor 12. Similarly, blade ring 48 is affixed to cylinder 20 and supports additional stator wheels which are interleaved with additional rotor wheels mounted on rotor 12, and blade ring 50 is affixed to cylinder 20 and supports further stator wheels which are interleaved with further rotor wheels mounted on rotor 12. It will be apparent that the pressure in first intermediate annular cavity 52 is less than the pressure in expansion chamber 40; that the pressure in second intermediate annular cavity 54 is less than the pressure in first intermediate cavity 52; and that the pressure is reduced again before the still-superheated steam reaches annular exhaust cavity 56. The steam in exhaust cavity 56 is emitted from turbine 10 via an exhaust port 58. A conduit 60 (illustrated only schematically) connects exhaust cavity 56 to an annular forward cavity 62, thereby maintaining the pressure in cavity 62 at substantially the same reduced level as in exhaust cavity 56. Labyrinth seal 6 is provided between shaft 14 and the rear end plate of cylinder 20, and a labyrinth seal 66 is provided between shaft 16 and the front end plate of cylinder 20.
A typical power plant in which turbine 10 may be used will now be described. Shaft 14 is connected by a coupling 68 to the shaft 70 of a low pressure turbine (not illustrated) which receives the steam from exhaust port 58. The other end of shaft 70, after emerging from the low-pressure turbine, is coupled, to an electrical generator (not illustrated). At the front end of turbine 10, shaft 16 is connected by a coupling 72 to the shaft 74 of a pump (not illustrated) which supplies lubricating oil to the bearing. A thrust bearing (not illustrated) maintains the axial location of the rotor with respect to the cylinder and transmits axial forces on the rotor to the plant foundation.
With continuing reference to FIG. 1, a narrow passage 76 exists in the region of control stage 38 between the rotating and stationary elements, this passage being exposed to high pressure, high temperature steam. Passage 76 leads to a high pressure annular cavity 78 at the left (in FIG. 1A) of a portion of rotor 12 known in the art as a "high pressure dummy," identified in FIG. 1B with reference number 80. Dummy 80 has a plurality of annular seal projections 82. Facing dummy 80, cylinder 20 is provided with a high pressure seal mounting portion 84 having annular grooves 86. Seal members which cooperate with dummy 80 are not illustrated in FIG. 1, but will be discussed below in conjunction with FIG. 2. To the right (in FIG. 1B) of dummy 80 is an intermediate pressure annular cavity 87, followed by a low pressure dummy 88 of rotor 12. Annular seal projections 90 extend toward a low pressure seal mounting portion 92 of cylinder 20. Portion 92 is provided with annular grooves 94 for mounting seal members, which are not illustrated in FIG. 1 but which will be discussed in conjunction with FIG. 2.
Turning next to FIG. 2, it will be seen that the seal projections 82 extending from high pressure dummy 80 have peripheral edge portions 96 and stepped side portions 98. In addition to the annular grooves 86, high pressure seal mounting portion 84 has annular grooves 100 which were not illustrated in FIG. 1 because of its reduced size. Seal members 102 are lodged into grooves 100 by caulking rings 104 (although not shown, grooves 100 have undercut regions in the manner of grooves 86 and rings 104 extend into the undercut regions). The tips or apex peripheries of seal members 102 are spaced apart from portions 96 by narrow gaps, typically about 20-30 mils (about 0.5 mm-0.8 mm). Seal rings 106 have foot portions 108 which are lodged into the annular grooves 86 by caulking rings 110. Seal rings 106 also have annular grooves 112, and an inner peripheral surface 114. Seal members 116 are wedged into grooves 112 by caulking rings 118. When they are in good condition, the tips or apex peripheries of seal members 116 are spaced apart from stepped side portion 98 by about 20-30 mils (about 0.5 mm-0.8 mm).
Between the seal projections 82, dummy 80 has annular grooves 120. Annular seal members or elements 122 are wedged into grooves 120 by caulking elements 124 and retainer rings 126. The tip or apex periphery of a seal element 122 is spaced apart from surface 114 by 20-30 mils (about 0.5 mm-0.8 mm). As is shown in FIG. 3, during installation seal element 122 and retainer ring 126 are first inserted into groove 120. Caulking element 124 is then pounded between them in the direction of arrow 128. Thereafter the projecting portion of retainer ring 126 is bent in the direction of arrow 130 to achieve the configuration illustrated in FIG. 2, which prevents element 124 from being dislodged due to centrifical force as the rotor 12 rotates. The rotor 12 of the specific turbine 10 shown in FIG. 1 has a maximum diameter of 211/4 inches (54 cm) and rotates at 3600 rpm during normal operation, and it will be appreciated that the centrifugal force is substantial.
Returning to FIG. 2, although the members 102 and the seal elements 122 extend in the radial direction with respect to the axis of rotor 12 (FIG. 1), the sealing arrangement shown in this figure is nevertheless known as an "axial clearance labyrinth seal assembly" because more than half of the sealing effect is provided by seal members 116, which extend axially.
Referring next to FIGS. 1 and 2 together, although an axial clearance labyrinth seal assembly has been described in detail only for high pressure dummy 80, an axial clearance labyrinth seal assembly is also employed at low pressure dummy 88. The only difference is that the radius of the various elements is larger at low pressure dummy 88, and the number of seal projections 90 (and the associated seal rings and sealing elements, as discussed with respect to FIG. 2) is less.
Since the various rotor wheels mounted on rotor 12 in FIG. 1 have fan-type blades, a torque is exerted on rotor 12 as the steam moves through turbine 10. However, the rotor wheels can also be likened to porous pistons, which are responsive to pressure differences. The pressure drop which occurs between gap 76 and cavity 56 is distributed between rotating and stationary blade rows. The pressure drop across the rotating blade rows imparts a force (acting to the left in FIG. 1) which must be balanced in the opposite direction. This is the purpose of the high pressure dummy 80 and low pressure dummy 88. During steady-state operation of turbine 10, with rotor 12 rotating at 3600 revolutions per minute and with the temperature stabilized, the pressure in cavity 78 is typically 600 pounds per square inch (about 4.14 megapascals, abbreviated MPa) and the temperature of the superheated steam is typically 790.degree. F. (421.degree. C.). The pressure and temperature drop to perhaps 295 PSI (2.03 MPa) at 765.degree. F. (407.degree. C.) in intermediate pressure cavity 87, and to perhaps 75 PSI at 745.degree. F. (401.degree. C.) in cavity 62. The effective diameters of high pressure dummy 80 and low pressure dummy 88 are selected to minimize the total axial force on rotor 12, with any unbalanced force being transmitted to the thrust bearing.
Like all things mechanical, turbine 10 is subject to wear. In particular, the seal members 102 and 112 in FIG. 2, along with seal elements 122, are subject to a harsh environment which erodes the members 102 and 112 and elements 122. Furthermore, it has been found that the desire to bring generating capacity rapidly back on-line, after it has been shut down for some reason, occasionally leads operating personnel to pay less than full attention to manufacturers' specifications for bringing turbine 10 up to speed at a relatively modest pace. When this occurs the rotating elements are heated more rapidly than the stationary elements, and the resulting differences in thermal expansion may damage seal members 102 and 112, and seal elements 122. Because the differential expansion is greater in the axial direction than in the radial direction, damage to the seal elements 122 is usually more extensive. Both normal erosion and damage due to failure to follow start-up specifications results in increased leakage to the seals at the high pressure dummy 80 and low pressure dummy 88, thereby reducing efficiency of the turbine. Moreover, the leakage alters the pressure ratios across the high pressure dummy 80 and low pressure dummy 88, thereby increasing the axial flow along on rotor 12 and creating an undesirable noise problem.
One conventional way to repair the seals associated with dummies 80 and 88 is as follows: After upper and lower shells 20a and 20b have been removed, both the high pressure dummy 80 and the low pressure dummy 88 are machined to a common radius, illustrated by dotted line 132 in FIG. 1. This leaves a stub, and an annular element (not shown) having disk portions with the desired radii for dummies 80 and 88 is shrunk-on. That is, the annular element, with an inside diameter similar to the outside diameter of the stub, is heated to high temperatures and slipped over the stub, and then allowed to contract into firm engagement with rotor 12. Additionally, the seal rings 106 are removed from cylinder 20, and seal mounting portions 84 and 92 are machined, for example to the depth illustrated by dotted line 134 in FIG. 2. Unlike the situation with rotor 12, portions 84 and 92 need not be machined to the same radius. Thereafter, arcuate elements (not illustrated) having radially extending seals similar to seal members 102 are bolted onto what is left of portions 84 and 92. These seals cooperate with the shrunk-on disk portions, but it will be apparent that the repair procedure is labor-intensive, time-consuming, and requires the procurement of large forgings for the shrunk-on disks. Furthermore, the engineering effort to design the annular element is very significant.
In a variation on the above-described repair procedure, the high pressure dummy 80 is machined down to one depth and the low pressure dummy 88 is machined to a lesser depth (i.e., a larger diameter). Weld build-up is then employed to increase the radius at high pressure dummy 80 and at low pressure dummy 88 to the necessary level.